Many investigations have been made on computer-to-plate (CTP) systems in which a printing plate is produced through laser beam scanning exposure based on digital signals. Among these, investigations on a lithographic printing plate precursor which does not necessitate development and can be attached, without any treatment after exposure, to a printing machine (i.e., a printing press) and used for printing have been made. This type of plate precursor is intended to attain further rationalization of platemaking and to mitigate problems concerning waste liquid treatment. For example, various techniques concerning CTP printing plates necessitating no development are described in Nihon Insatsu Gakkai-shi, Vol.36, pp.148-163 (1999).
One promising technique is a method utilizing ablation which comprises exposing a printing plate precursor with a solid high-power infrared laser, such as a semiconductor laser or YAG laser, to heat the exposed areas by the action of a light-to-heat converting agent (i.e., a compound capable of converting light into heat) which converts light into heat and to thereby cause those areas to decompose and evaporate.
Namely, the technique described above comprises forming a hydrophilic layer on a base (i.e., a substrate) having an oleophilic ink-receptive surface or ink-receptive layer and removing the hydrophilic layer by ablation.
In WO 94/18005 is described a printing plate produced by forming a crosslinked hydrophilic layer on an oleophilic laser light-absorbing layer and ablating the hydrophilic layer. This hydrophilic layer comprises poly(vinyl alcohol) crosslinked with a hydrolyzate of tetraethoxysilicon and containing titanium dioxide particles, and is intended to have improved film strength. Although this technique has brought about improved press life, it is insufficient in the property of not causing staining and a further improvement has been required.
In WO 98/40212 and WO 99/19143 is described a lithographic printing plate precursor which comprises a base, an ink-receptive layer formed thereon, and a hydrophilic layer formed thereon comprising as the main component a colloidal oxide, such as silica, crosslinked with a crosslinking agent such as aminopropyltriethoxysilane and which can be attached to a printing machine (i.e., a printing press) without undergoing development. This hydrophilic layer is intended to have a minimal amount of hydrocarbon groups for enhancing the property of not causing staining and to have improved press life due to the crosslinking of a colloid with a crosslinking agent. However, the impression capability of this printing plate is several thousand impressions, which has been still insufficient.
The heat-sensitive lithographic printing plate precursor utilizing ablation has the problem that it does not provide both of improved printing durability (i.e., press life) and property of not causing staining. In addition, this printing plate precursor has had the following drawbacks. Since ablation debris fly off to stain the laser-exposing apparatus and optical system, it is necessary to provide these apparatus with an ablation debris trapping apparatus. Furthermore, even with the trapping apparatus, it is difficult to sufficiently eliminate the staining.
As a result of extensive investigations, it was found that a heat-sensitive lithographic printing plate precursor which gives a plating plate having excellent printing durability and causing no staining and is inhibited from causing ablation debris flying is obtained by forming a hydrophilic layer containing a colloid of an oxide or hydroxide of at least one element selected from the group consisting of beryllium, magnesium, aluminum, silicon, titanium, boron, germanium, tin, zirconium, iron, vanadium, antimony, and the transition metals and a water-soluble overcoat layer on a base having an ink-receptive surface or coated with an ink-receptive layer (see Japanese Patent Application (Laid-Open) No. 96936/2001).
However, this heat-sensitive lithographic printing plate precursor still has a problem that the printing plate has insufficient ink receptivity in the beginning of printing and necessitates a large amount of spoilage before complete ink reception.